Because piston rings for automobile engines are required to have excellent slidability, piston rings made of steel such as silicon-chromium steel (SWOSC-V) and martensitic stainless steel [SUS440B (17% Cr), SUS420J2 (13% Cr), etc.] are usually surface-treated. Among surface treatments, economically advantageous nitriding is conveniently used for martensitic stainless steel, high-alloy steel. However, because silicon-chromium steel, low-alloy steel, is not sufficiently nitrided, ion plating of chromium nitride (CrN) and chromium plating are used.
Piston ring materials are also required to have not only the above slidability, but also heat resistance for keeping spring characteristics, thermal conductivity for cooling pistons, wear resistance for side surfaces, and good workability (hot- and cold-workability). From the aspects of thermal conductivity, workability and cost, heat resistance (heat sag resistance) is desirably improved by using low-alloy steel, to keep spring characteristics and have good wear resistance on side surfaces. Particularly with respect to the wear resistance of side surfaces, the formation of a desired nitrided layer on low-alloy steel would provide larger economic and environmental effects than those of other surface treatments.
To provide steel capable of being formed by cold rolling and nitriding into light-weight, low-cost piston rings having high performance, JP 2000-282177 A discloses steel comprising by weight 0.50-0.80% of C, 1.55-3.00% of Si, 0.20-1.50% of Mn, 0.60-2.00% of Cr, and 0.05-1.00% of V, the balance being Fe and inevitable impurities, the maximum thickness of oxide inclusions containing 80% or more of Al or Si being 30 μm or less in a rolling-direction cross section including a center portion of the steel. This material is obtained by adding V to the above-described silicon-chromium steel (SWOSC-V), with high upper limits of other elements. With respect to a nitrided layer, it discloses a surface layer having hardness of 903-1191 HV, and a hardened layer having depth (not defined) of 0.26-0.32 mm, though not disclosing nitriding conditions, etc.
To provide an inexpensive piston ring having excellent slidability without wearing ring grooves of an aluminum alloy piston, JP 2005-61389 A discloses a piston ring having a hard coating on an outer peripheral surface of a base material, and a chemical conversion coating on at least one of upper and lower side surfaces; the base material comprising by mass 0.50-0.70% of C, 1.65-2.50% of Si, 0.50-1.50% of Mn, 0.50-3.00% of Cr, and 0.05-0.50% of V, the balance being Fe and inevitable impurities; and the hard coating being formed by PVD or high-velocity oxy-fuel thermal spraying. It further discloses that when this base material is nitrided, it has surface hardness of less than 1000 HV0.1 and 700 HV0.1 or more, and that even after nitriding, a good chemical conversion coating is formed on its side surfaces. However, nitriding conditions and nitriding depth, etc. are not disclosed.
Though materials for the piston rings of JP 2000-282177 A and JP 2005-61389 A are categorized in low-alloy steel, the amount of alloy elements are slightly larger than those in so-called silicon-chromium steel, so that they are poorer than silicon-chromium steel in thermal conductivity. At present, there is no low-alloy steel for piston rings, which is better than silicon-chromium steel in thermal conductivity and the formability of a nitrided layer.